The present invention relates to a process for manufacturing a polycarbonate having few foreign materials. More specifically, this invention relates to a process for manufacturing a polycarbonate having few foreign materials by using a polymer filter.
Generally, foreign materials in polycarbonates are classified into foreign materials got mixed from raw materials or from the outside of a reaction system, and those generated in a reaction apparatus or in a passage for a highly viscous material obtained by reaction. Against the former, a preventing means for contamination comprises the use of a filter for filtering foreign materials in raw materials or the improvement of closeness (tightness) of the reaction system. On the other hand, against the latter, foreign materials are removed by using a filtration filter directly before the highly viscous material is processed into a desired form.
However, depending on the temperature or viscosity of a melting polymer to be filtered, the retained particle diameter of a filtration filter or the amount of treated materials, deterioration attributable to retention such as discoloring, cross-linking, gel generation would occur in a filtration filter, exerting great influence on the product quality.
Recently, especially in a polycarbonate used for optical applications such as DVD, MO or CD-R, which require high density and high accuracy, problems of foreign materials, discoloring, and gel generation have direct influence on optical properties such as block error rate, or on mechanical properties such as tensile strength, flexural strength and toughness of the final products, and these problems therefore are serious. Further, because gels characteristically change their shapes, those having a size even larger than the retained particle diameter of a filter would pass the filter, in some case, and this causes extremely severe problems.
The object of the present invention is to provide a process for solving problems of the above-mentioned prior arts, efficiently removing foreign materials by using a polymer filter, and suppressing the occurrence of discoloring, crosslinking and gel formation in the filter, at the same time, in order to manufacture a polycarbonate having excellent quality.
The present invention comprises the following.
1. A filtration apparatus for a polycarbonate comprising as constituent elements:
(a) a filtration container having a polymer inlet passage and an opening;
(b) a flange plate interfitting with the opening;
(c) one or more leaf disc-type filters for filtering a polymer introduced into the filtration container through the polymer inlet passage;
(d) a filter presser for fixing the leaf disc-type filters to each other and applying sealing pressure on the junction parts of the leaf disc-type filters; and
(e) a center pole placed on a space formed by the inner circumferences of the leaf disc-type filters, for collecting the polymer filtered by the leaf disc-type filters and for discharging it outside the filtration apparatus, and integrated with the filter presser.
2. A filtration apparatus according to above 1 characterized in that the center pole penetrates the flange plate, the flange plate has a sealing part for preventing the intrusion of a polymer down along the center pole into the penetration part of the center pole on the filtration container side of the flange plate, and the flange plate has a fixture for fixing the center pole with the flange plate on the side opposite to the filteration container of the flange plate.
3. A filtration apparatus according to above 1 or 2 characterized in that the center pole has a polymer flow passage through the inner side thereof on the portion corresponding to the section where the center pole penetrates the flange plate.
4. A filtration apparatus according to any one of above 1 to 3 characterized in that the shape of the cross section of the center pole is substantially a polygon or star shape having plural apexes and the length of each portion where the center pole contact with the inner circumference of the leaf disc-type filter is 3 mm or less on the cross section.
5. A filtration apparatus according to any one of above 1 to 4 characterized in that cross-sectional area of polymer flow passage which form between the center pole and the inner circumferences of a leaf disc-type filter increases continuously as it goes downstream along the polymer flow.
6. A filtration apparatus according to any one of above 1 to 5 characterized in that the boundary formed between the surface (a) substantially perpendicular to the center pole which is on the filtration container side of the flange plate, and the surface other than this surface (a) is a curved surface having a curvature of 1 mm or more.
7. A filtration apparatus according to any one of above 1 to 6 characterized in that the polycarbonate has a viscosity average molecular weight of 10,000 or more.
8. A filtration apparatus according to any one of above 1 to 7 characterized in that the polycarbonate is manufactured by polymerizing an aromatic diol compound and a carbonic acid diester compound in the presence or absence of a catalyst.
9. A process for manufacturing a polycarbonate characterized in that a filtration apparatus according to any one of above 1 to 8 is used for filtering the polymer.
It was made clear that, being different from polyesters such as polyethylene terephthalate, when a polycarbonate is heated for a long time even in non-oxygen atmosphere at such a high temperature as is enough to melt a polycarbonate, it forms a crosslinked structure, and eventually a material, called gel, having a viscoelastic behavior different from that of polycarbonate, and an insoluble substance, and the hue is also deteriorated.
For this reason, it is important to reduce, as much as possible, sections where polymer stagnates, which is called dead space, in the filtration of a polycarbonate.
Filtration apparatuses conventionally used for filtering a polymer include those of a candle type and a plate type besides a leaf disc type, but the leaf disc-type filtration apparatus is preferred for filtering a polycarbonate from the view points of the above-mentioned dead space, filtration area and the exchangeability of filters.
However, in the leaf disc type filtration apparatus, because plural toroidal filters to be used are stacked up so that the polymer filtered through them is collected and discharged outside the filtration apparatus via a center pole (a hollow or grooved rod), sealing members placed on the inner circumference of the toroidal filters must be closely joined to each other so that the unfiltered polymer existing outside the filters does not mix with the filtered polymer existing inside the filters or the center pole. Hence, in a conventional leaf disc-type filtration apparatus, a filter presser is placed at an end of the stacked leaf disc-type filters, a center pole is placed through a space formed by the inner circumference of the stacked leaf disc filters, and these elements are tightened with fixtures such as bolts and nuts in order to assemble the filtration apparatus.
Accordingly, the bolts and nuts used for tightening come to contact with the molten polymer, and gaps formed at tightening parts act as dead space, resulting with problems such as deterioration of the hue of the obtained polymer and gel formation.
According to the present invention, the filter presser and the center pole have an integral structure, whereby dead space is not formed, and bolts for tightening the center pole and the filters are not needed. Further, when nuts are made not to contact with a molten polymer, dead space is not formed, and the deterioration of polymer is prevented to make possible to produce a polycarbonate having excellent quality. Here, xe2x80x9cthe filter presser and the center pole have an integral structurexe2x80x9d means that the filter presser and the center pole are not separable into plural parts.
In the present invention, it is preferable that the shape of cross section of the center pole in the direction perpendicular to its axis where the center pole contacts with the inner circumference of a leaf disc-type filter is substantially a polygon or a star shape having plural apexes. Such a shape allows that the filtered polymer flows out toward the outside of the system through a polymer passage formed by the outer circumference of the center pole (that is, the circumference formed by linking the apexes of the external polygonal or star-like shape in the cross-section of the center pole part in the direction perpendicular to its axis), the grooves carved on the center pole part, and the inner circumferences of the leaf disc-type filters, and therefore, dead space formed to a considerably lesser extent than in the case of a hollow center pole. Further, the area where the inner circumferences of the leaf disc-type filters are in contact with the outer circumference of the center pole is small. Thus, the dead space generation on the contacting surfaces can be avoided. In this regard, the length of each portion where the center pole contact with the inner circumference of the leaf disc-type filter is more preferably 3 mm or less on the cross section.
Further, while the amount of the filtered polymer flowing through the polymer passage of the center pole increases as the flow approaches the flange plate, that is, the amount increases as the flow of the polymer goes downstream, it is preferable that the pressure loss caused by the flow of the polymer is made small in order to realize good filtration. For this purpose, the cross sectional area of polymer flow passage which form between the center pole and the inner circumferences of a leaf disc-type filter preferably increases continuously as the polymer flow goes downstream along the polymer flow. In concrete, for example, the depth of the grooves formed on the center pole is preferably made deeper as the polymer flow approaches the flange plate.
In the present invention, the center pole preferably has a hollow polymer passage in the area where it penetrates the flange plate. By this structure, it becomes possible that fixtures such as a fixture nut for the center pole are prevented from contact with polymer.
In the present invention, when a polymer flows inside of the filtration apparatus formed by the filtration container and the flange plate, the polymer flow direction is changed drastically near the juncture of the filtration container and the flange plate, and dead space is apt to be generated there. So, a corner having a curved surface of a curvature of 1 mm or more is preferably formed on the flange plate side.